Paper advancing system for high speed printers

ABSTRACT

A method and associated apparatus for quickly and accurately advancing paper between the platen and printing mechanism on a line-by-line basis in a high speed printer for printing lines of text including a platen and a printing mechanism disposed adjacent the platen. The method comprises the steps of, threading the paper from the supply input through a powered first drive mechanism disposed before the platen and printing mechanism, through a frictional gripping mechanism disposed adjacent the platen between the platen and the first drive mechanism and adapted for frictionally gripping the paper under a gripping force sufficient to prevent advancement beyond the frictional gripping mechanism by a pushing force on the paper while permitting the paper to be pulled through the frictional gripping mechanism by a second drive mechanism, between the platen and printing mechanism and into engagement with a powered second drive mechanism disposed after the platen and printing mechanism and adapted for gripping and rapidly moving the paper a feed distance; using the powered first drive mechanism to receive the paper and advance at least one dot line feed distance of paper to form a supply loop between the powered first drive mechanism and the frictional gripping mechanism; and, at the time for advancing the paper by the feed distance, which may be while the first drive mechanism is still operating, using the powered second drive mechanism to pull the paper through the frictional gripping mechanism the feed distance from the supply loop.

This is a continuation-in-part of copending application Ser. No. 230,457filed on Aug. 10, 1988 now U.S. Pat. No. 4,896,980.

BACKGROUND OF THE INVENTION

The present invention relates to paper feeders for computer-drivenprinters, and the like, and, more particularly, to high speed paperadvancing apparatus for moving paper on a line-by-line basis from asupply input between the platen and printing mechanism in a printerprinting lines of text and including a platen and a printing mechanismdisposed adjacent the platen comprising, powered lower drive meansdisposed below the platen and printing mechanism for receiving the paperfrom the supply input and for forming a supply loop containing paper forat least one dot line feed distance between the powered lower drivemeans and the platen and printing mechanism; powered upper drive meansdisposed above the platen and printing mechanism for gripping andrapidly moving the paper the feed distance from the supply loop betweenthe powered lower drive means and the platen and printing mechanism;and, control logic means operably connected to the powered lower drivemeans and the powered upper drive means for causing the lower drivemeans to receive the paper from the supply input and form the supplyloop during the time the printing apparatus is printing a line and forcausing the upper drive means to move the paper from the supply loop thefeed distance when the printing apparatus has completed printing a lineand requires the paper to be advanced the feed distance in order toprint a next line.

Most high speed alphanumeric printers used in association with computersto be driven thereby employ so-called "fanfold" or continuous paperhaving removable edges on either side containing equally spaced driveholes therein. A so-called "tractor feed" mechanism is then employed topull the paper through the printer. A typical prior art approach topaper advancing with a tractor feed mechanism is depicted in FIGS. 1 and2.

As can be seen, a driven tractor feed 10 is placed on either side of thepaper 12 so that the drive pegs 14 engaged the holes 16 in the removableedges 18 of the paper 12. The paper is fed upward vertically from asupply stack (not shown), between the print station 20 and platen 22,and over the tractor feeds 10. The print station 20 can comprise a dotmatrix printhead, a shuttle printhead, a "daisywheel" printhead, or thelike. The paper is typically held against the pegs 14 of the tractorfeeds 10 with a spring-loaded pressure grip (not shown). Typically, oneroller 24 of one of the tractor feeds 10 is connected to the platen 22by a gear train (symbolized by the dashed line 26) to be driven incombination therewith. The other roller 28 over which the tractor belt30 is stretched is an idler roller. The two drive rollers 24 areinterconnected by a shaft. Thus, as the platen 22 is moved in the "linefeed" mode, the two drive rollers 24 are rotated. This, in turn, rotatesthe tractor belt 30 which pulls the paper 12 up to the proper positionfor printing the next line of text.

Tractor feed systems such as that shown in FIGS. 1 and 2 work reasonablywell for slow speed operation. As long as the sequence of operation forpaper advancement as described above take place slowly enough, thestarting torque requirements placed on the platen drive motor (notshown) are low and sufficient power can be developed to pull the span ofpaper 12 extending downward (under the effects of gravity and friction)from the tractor feeds 10 to the supply stack. In slow speed operation,there is a virtual constant "downward" pressure on all the componentsand clearances provided to prevent binding of parts and occurringnaturally from wear to not cause problems. Likewise, the concept of"braking" and "overshoot" are meaningless. In high speed operation,however, these previously ignorable factors suddenly take on monumentalproportions leading to non-operability of the paper advancing system. Ascan be appreciated by those skilled in the art, the paper advancingsystem is the potential "weak link" in a high speed printer; that is, ifthe paper cannot be advanced line-by-line to keep up with the printingmechanism, the printing mechanism must be slowed down below itspotential.

When trying to achieve paper advance steps such as 1/72" in 1 ms or5/72" in 2 ms as is required to meet the printing speed capabilities ofcertain contemporary shuttle printers, for example, the above-describedtractor feed approach of pulling the paper over the platen and past theprint station 20 fails dismally. If starting torques sufficient toaccelerate the paper 12 to the required speed are applied, the holes 16can be ripped out of the paper 12 as a result of the high startinginertia of the mass of paper that must be moved each time. Withso-called "laser cut" paper where the lightly attached edges 18 areintended to break off smoothly, the edges 18 may just pull off andadvance while the paper 12 stays put. Multi-ply paper also causesdifficulties because of its mass. In those instances where the paper isbrought up to speed without incident, it may not stop in time because ofthe same high mass (relatively speaking) in motion. Without a positivebrake, the paper 12 between the print station 20 and tractor feeds 10may overshoot slightly and then settle back down to its proper positionhanging from the tractor feeds 10. This, of course, can result in acurved line of text with a high point at the beginning.

Wherefore, it is the object of the present invention to provide a paperadvancing system for use in high speed printers, and the like, which canadvance continuous paper on a line-by-line basis quickly, accurately,and without overshoot, or the like.

Other objects and benefits of the present invention will become apparentfrom the description which follows hereinafter when taken in conjunctionwith the drawing figures which accompany it.

SUMMARY

The foregoing objects have been achieved in a printer for printing linesof text including a platen and a printing mechanism disposed adjacentthe platen, by the improved paper advancing apparatus of the presentinvention for moving paper on a line-by-line basis from a supply inputbetween the platen and printing mechanism comprising, powered lowerdrive means disposed below the platen and printing mechanism forreceiving the paper from the supply input and for forming a supply loopcontaining paper for at least one dot line feed distance between thepowered lower drive means and the platen and printing mechanism; poweredupper drive means disposed above the platen and printing mechanism forgripping and rapidly moving the paper the feed distance from the supplyloop between the powered lower drive means and the platen and printingmechanism; and, control logic means operably connected to the poweredlower drive means and the powered upper drive means for causing thelower drive means to receive the paper from the supply input and formthe supply loop during the time the printing apparatus is printing aline and for causing the upper drive means to move the paper from thesupply loop the feed distance when the printing apparatus has completedprinting a line and requires the paper to be advanced the feed distancein order to print a next line.

In one version, the paper is continuous paper and the powered lowerdrive means comprises a first tractor feed mechanism disposed forlifting the paper from a supply stack and for advancing it ahead of thefirst tractor feed mechanism and frictional gripping means disposedadjacent the platen between the platen and the tractor feed mechanismfor frictionally gripping the paper under a gripping force sufficient toprevent advancement beyond the frictional gripping means by the firsttractor feed mechanism while permitting the paper to be pulled throughthe frictional gripping means by the upper drive means. In the preferredembodiment, the powered lower drive means additionally comprises a firstdrive motor operably connected to the tractor feed mechanism and thecontrol logic means wherein the first drive motor is a stepping motor.

The preferred frictional gripping means comprises a smooth surfacedguide bar disposed parallel to the platen and a spring metal pressureplate disposed parallel to the guide bar, the pressure plate having asmooth pressure edge which is curved in cross section and bears againstthe guide to releasably grip the paper therebetween.

The preferred powered upper drive means comprises, a pair of feedrollers mounted for rotation and disposed parallel to the platenadjacent respective side edges of the paper; a pair of pressure idlerrollers mounted for rotation and disposed parallel to respective ones ofthe feed rollers in contacting relationship thereto; means for urgingthe pressure idler rollers against the feed rollers to create a grippingforce therebetween for frictionally gripping the paper under a grippingforce sufficient to pull the paper through the frictional grippingmeans; and, a second drive motor operably connected to the feed rollerand the control logic means wherein the second drive motor is a D.C.motor. Preferably, there are also position sensor means for developing asignal reflecting the rotational position of the second drive motoroperably connected to the control logic means.

Also in the preferred embodiment, there is an electromagnetic brakedisposed between the feed roller and the end of the drive shaft of thesecond drive motor as well as a backlash coupling mechanism having anarc of freeplay motion disposed between the feed roller and the end ofthe drive shaft of the second drive motor, wherein the control logicmeans includes means for backing up the second drive motor to a pointtowards the beginning of the freeplay during the time the printer isprinting a line of text and for starting the second drive motor in aforward direction at a time prior to the time the upper drive means isto move the paper from the supply loop the feed distance such that thefreeplay is exhausted exactly at the moment the upper drive means is tomove the paper from the supply loop the feed distance.

In an embodiment for use with single sheet paper, the powered lowerdrive means comprises, a feed mechanism disposed for receiving a sheetof paper from a supply input and for advancing it ahead of the feedmechanism; pressure roller means for holding a sheet of paper againstthe feed mechanism for advancement thereby; and, frictional grippingmeans disposed adjacent the platen between the platen and the tractorfeed mechanism for frictionally gripping the paper under a grippingforce sufficient to prevent advancement beyond the frictional grippingmeans by the feed mechanism while permitting the paper to be pulledthrough the frictional gripping means by the upper drive means.

In an alternate embodiment for use with continuous paper, the poweredupper drive means comprises a second tractor feed mechanism disposed forgripping the paper and pulling the paper through the frictional grippingmeans.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified drawing depicting a prior art tractor feed typeof paper advancing system.

FIG. 2 is a detailed side view of the tractor feed paper advancingsystem of FIG. 1.

FIG. 3 is a detailed side view of the high speed paper advancing systemof the present system.

FIG. 4 is a partially cutaway front view of the feed roller driveassembly of the present invention.

FIG. 5 is a front view of the backlash coupling device employed in thepreferred embodiment of the present invention.

FIG. 6 is a cutaway drawing of the backlash coupling device of FIG. 5 inthe plane VI-VI.

FIG. 7 is a motion and timing diagram of the operation of the presentinvention.

FIG. 8 is a detailed side view of the high speed paper advancing systemof the present system in an alternate embodiment intended for use withcut paper sheets as fed from a sheet feeder.

FIG. 9 is a detailed side view of a portion of the high speed paperadvancing system of the present system in another alternate embodimentwhere the upper feed rollers have been replaced by a tractor feedmechanism.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Before beginning the description of the present invention, the conceptof line feed as employed in the description and claims which followshould be clarified. In a high speed dot matrix printer of the shuttlevariety as wherein the present invention is primarily intended to beused, a single line of dots may be produced by multiple print headsdisposed in side-by-side relationship. After a line is printed, thepaper is advanced by "one dot line", which is the distance betweenvertically spaced dots. This distance is typically in the order of 1/72inch. The typical "line feed" as thought of in alpha-numeric printingrefers to the vertical distance between lines (i.e. rows) of characters.This distance is much larger than the one dot line distance, e.g.typically in the order of 1/6 inch. There is also a "fractional linefeed" which is typically in the order of 5/72 inch. While the presentinvention as described and claimed herein refers to one dot line ofpaper advancement, the invention could be used with a high speedalpha-numeric printer, in which case the one dot line referred to would,in fact, be one line feed from one line of characters to the next. It isthe inventors' intent that the present application and the claimsappended thereto be accorded a breadth in keeping with the scope andspirit of the invention as disclosed therein and that they not belimited by the use of particular language with respect to line feedingand the distances traversed by the paper in being so fed.

The present invention is based on the proposition of accomplishing thetask of lifting the large mass of paper extending from adjacent itspoint of use to the supply stack during the extended period of time(relatively speaking) that exists while a line of text is being printedand then moving only a small mass through the line feed distance duringthe time between lines. This is accomplished by the paper advancingmechanism generally indicated as 32 in FIG. 3. Details of the feedroller drive assembly thereof are shown in FIG. 4. The paper advancingmechanism 32 of the present invention as applied to the printing ofcontinuous, fan fold paper comprises three major components--a tractorfeed lifting mechanism, generally indicated as 34, a lower paper guideassembly, generally indicated as 36, and a feed roller assembly,generally indicated as 38.

The tractor feed lifting mechanism 34 comprises a horizontally disposedtractor feed mechanism 10' generally as described above. A pair of driverollers 24 are located to be on either side of the paper 12. The driverollers 24 are interconnected by a shaft 40 so as to move incombination. One end of the shaft 40 has a drive gear 42 concentricallymounted thereon. The drive gear 42 is operably connected to be driven bya pinion gear 44 carried by the drive shaft 46 of a lower stepping motor48. The stepping motor 48 is controlled by control logic 50. Thus,independently and at the proper time (to be described shortly), bystepping the motor 48, the control logic 50 can independently turn thedrive rollers 24, tractor belts 30, and idler rollers 28 in combinationto lift the paper 12 from the supply stack (not shown) and push ithorizontally toward the lower paper guide assembly 36 as indicated bythe arrow 52. As will be appreciated by those skilled in the art, otherfunctionally equivalent types of motors could, of course, be employedfor the motor 48, e.g. D.C. motors, rotary or linear solenoid motors,etc.

The lower paper guide assembly 36 is fastened adjacent the platen 22'.Note that the platen 22' is a non-rotating bar and not a roller as inthe prior art since the present invention is not an adaptation of oldtypewriter paper feed mechanisms as are most of the prior art paperadvancing mechanisms. A smooth surfaced guide bar 54 extends across thewidth of the platen 22' and parallel thereto just under the printingface 56 thereof. A supplemental bar 58 is positioned below and behindthe guide bar 54. In the event of a malfunction, the supplemental bar 58prevents the paper 12 from lifting off of the drive pegs 14 and movinginto parts of the paper advancing mechanism 32 where it should not be. Aspring metal pressure plate 60 is disposed parallel to the guide bar 54across the width of the paper 12. The pressure plate is generallyL-shaped in cross section. There is a horizontal portion 62 and avertical portion 64. The ends of the bars 54 and 58 and the horizontalportion are attached to a pair of end plates 66 by means of which thelower paper guide assembly 36 can be removably and adjustably attachedto the sidewalls of a printer such as by cap screws 68. The verticalportion 64 of the pressure plate 60 is free to flex about its lower end70. The upper edge of the vertical portion 64 terminates in a smoothpressure edge 72 which is curved in cross section. The edge 72 bearsagainst the guide bar 54 and releasably grips the paper therebetween.This will be returned to shortly.

The feed roller assembly 38 is best understood through simultaneousreference to FIGS. 3 and 4. A pair of rubber feed rollers 74 are mountedfor rotation disposed above and parallel to the platen 22' adjacent therespective ends thereof so as to grip the side edges of the paper 12.The front edges of the rollers 74 are tangent to a plane passingsubstantially vertically through the printing face 56 thereof. The feedrollers 74 are resiliently slip-mounted on a shaft 47 by means of afriction or magnetic clutch (not shown). Preferably, the surface of thefeed rollers 74 has an abrasive thereon to provide positive gripping ofthe paper for driving purposes. A pair of rubber pressure idler rollers76 are mounted for rotation disposed parallel to the feed rollers 74with a facing edge thereof in contact with the front edges of therollers 74. The pressure idler rollers 76 are biased towards the feedrollers 74, such as by a pair of springs 78, so as to create a pinchingforce between the rollers 74, 76. As a consequence, paper 12 passingbetween the rollers 74, 76 is gripped between them and can only be movedby their combined rotation. Alternately, feed rollers 74 may bepositively rotated by shaft 47 with slip, when required, being betweenthe feed rollers 74 and the paper 12. Optionally, a spring pressureplate could be used in lieu of the feed rollers 76 to hold the paperagainst the rollers 74 for driving purposes.

One end of the shaft 47 carrying the drive rollers 74 is connected (bymeans to be described shortly) to the drive shaft 46 of an upper D.C.motor 80. Again, as will be appreciated by those skilled in the art,other functionally equivalent types of motors could, of course, beemployed for the motor 80, e.g. stepping motors, rotary or linearsolenoid motors, etc. The motor 80 is also operably connected to thecontrol logic 50 to be operated thereby. Between the shaft 46 of themotor 80 and the end of the shaft 47 there is an optically encodedposition disk 82, a backlash mechanism 84 (optional but preferred), andan electromagnetic brake 86 connected to ground potential. An opticalsensor 88 is mounted to the motor 80 and connected to the control logic50. The disk 82 is graduated to provide twenty sensible graduations perstep of the motor 80. The sensor 88 and disk 82 are according totechniques well known in the art for providing a feedback of therotational position of the motor 80 as it is stepped so as to allow thecontrol logic 50 to accurately control the acceleration and stopping ofthe motor 80 in the novel manner of the present invention to bedescribed shortly. Per se, the sensor 88 and disk 82 form no part of thenovelty of the present invention and, therefore, in the interest ofsimplicity and the avoidance of redundancy, they will not be describedfurther. The same is true for the electromagnetic brake 86.

The backlash coupling mechanism 84, while not absolutely necessary forthe operation of the present invention, provides superior operation andhelps in the avoidance of possible problems and is, therefore,preferred. For example, when introduced in the position shown in FIG. 4,it will reduce paper line feed time approximately 25% as compared to thesame system without it. As can be seen from the detailed drawings ofFIGS. 5 and 6, it is similar in construction to a universal joint, i.e.comprising a driving portion 90 and a driven portion 92 each havinghorizontal drive fingers 94 at 180 degrees from one another with thedrive fingers 94 of the driving portion 90 oriented 90 degrees from thedrive fingers 94 of the driven portion 92. Instead of being connectedtogether by rotational pins as in a universal joint (which would preventrotational displacement, i.e. backlash, between the driving and drivenportions 90, 92) the arcs between the adjacent drive fingers 94 areoccupied by an impact member 96 comprising arcuate pieces of a highimpact elastomeric material such as many plastics well known in the art.The arcuate pieces of the impact member 96 are of an arc distance lessthan the arc distance between the adjacent drive fingers 94 to providean arcuate movement area of freeplay. This difference is the "backlashangle" as indicated in FIG. 6. In the preferred embodiment of thecoupling mechanism 84, the backlash angle is provided to separate theinertia of the upper stepping motor 80 from the inertia of the feedrollers 74 driving the paper 12 to its new line position. This angularseparation allows the upper stepping motor 80 to undergo a higherinitial acceleration which, upon the subsequent impact of the drivingportion 90 with the driven portion 92 through the impact member 96,creates a high instantaneous line feed acceleration of the feed rollers74. To use an analogy, it acts like a hammer driving a nail. One swingsthe hammer, one does not hold the hammer against the nail and push. Thestepping motor 80 only has to accelerate its own mass and then thedriving portion 90 of the coupling mechanism 84 strikes the drivenportion 92 with an impact that applies a large step acceleration forceto the feed rollers 74. After each line feed has been completed, theelectromagnetic brake is activated to stop the movement of the driverollers 74 and the paper being driven thereby. Then, the upper steppingmotor 80 is reversed through the freeplay area the proper number ofsteps towards the beginning thereof to restore the majority of thebacklash angle. Like the lifting of the paper weight from the supplystack, this occurs during the printing portion of the cycle. Note thatthe entire backlash angle is not traversed as to do so might result inmoving the paper during the printing process. Note also that the timeinterval required to accelerate the stepping motor 80 through thebacklash angle also occurs during the printing portion of the cycle;that is, just prior to the time for line feed, the control logic 50starts the stepping motor 80 through the backlash angle. This is timedsuch that the impact of the driving portion 90 with the driven portion92 occurs exactly at the instant that line feed is desired. As a result,the line feed loop 98 is driven virtually instantaneously to its newposition. As a result, it was found that paper advancement could takeplace as follows--1/72" in less than 0.7 ms, 5/72 " in 2 ms, and 1/6" inless than 6 ms. In passing, it should be noted that a friction clutch,or the like, is included in the resilient slip mounting of the rollers74 on the shaft 47 so as to allow the feed rollers 74 to stop rotationwhen the loop 98 is removed. This prevents the rollers from spinningagainst the paper and the consequent loading of its surface.

Returning now to FIG. 3 with particularity, the manner of operation ofthe above-described apparatus under the control of the control logic 50will now be described in detail. During the printing of a line of texton the paper 12, the logic 50 steps the lower stepping motor 48 to liftthe paper 12 from the supply stack and urge a next line feed portionhorizontally forward as indicated by the arrow 52. At this time, thepaper is being gripped between the pressure edge 72 of the pressureplate 60 and the guide bar 54 and, therefore cannot move forward alongits path of travel beyond that point. Accordingly, the paper between thetractor feed mechanism 10' and the above-described point of gripping isforced into the junction of the horizontal and vertical portions 62, 64of the pressure plate 60 thus forming a line feed loop of the paper 12as indicated by the dashed line 98. The paper 12 is only advanced onedot line feed distance by the motor 80 and control logic 50 and,therefore, the line feed loop 98 contains only that amount of the paper12, i.e. a very low mass. In a tested embodiment of the presentinvention, the pressure of the edge 72 on the guide bar 54 is such thatup to 1/3" of paper can be formed into the loop 98 without forcing paperpast the platen 22'.

When it is time for a line feed of the paper 12 following the end ofprinting of a line of text, the control logic 50 steps the upperstepping motor 80 to lift the small mass of the paper 12 between thegripping point of contact between the rollers 74, 76 and the line feedloop 98 through the gripping force which exists between the pressureedge 72 of the pressure plate 60 and the guide bar 54. Because of thelow mass being moved, there can be no overshoot and the paper 12 comesquickly to its new position as depicted by the solid lines in FIG. 3with the paper 12 stretched across the platen. The braking effect of thegripping force which exists between the pressure edge 72 of the pressureplate 60 and the guide bar 54 prevents the line feed loop 98 from"creeping" and affecting the paper position during printing.

As those skilled in the art will appreciate, while the tractor feedmechanism of the present invention as described hereinbefore is disposedto lift the paper and advance it in a horizontal direction to form theline feed loop 98, and such an orientation is preferred, it could beoriented to lift the paper vertically and also advance it vertically.The important aspect of the present invention is the providing of thefrictional gripping point between the tractor feed and the platen sothat advanced paper cannot move beyond that point and, therefore,"bunches up" to form the line feed loop which is subsequently pulledthrough the frictional gripping point.

While the principal application of the present invention is withcontinuous form paper, as mentioned earlier, there may be instanceswhere the benefits thereof can be applied to a single sheet printerwherein the paper is input on a sheet-by-sheet basis from a sheetfeeder. In such applications, the present invention could be configuredas shown in FIG. 8 wherein it is generally indicated as 32'. Basically,all that has to be done is to replace the tractor feed lifting mechanism34 with a sheet feeding mechanism such as that generally indicated as100. The mechanism 100 is representative only and other types of papergripping drives could, of course, be employed for the purpose. Themechanism comprises a horizontally disposed belt feed mechanism 102generally as described above. As in the tractor feed 10', a pair ofdrive rollers 24 are located to be on either side of the paper 12. Thedrive rollers 24 are interconnected by a shaft 40 so as to move incombination. One end of the shaft 40 has a drive gear 42 concentricallymounted thereon. The drive gear 42 is operably connected to be driven bya pinion gear 44 carried by the drive shaft 46 of a lower stepping motor48. The stepping motor 48 is controlled by control logic 50. A pair ofnon-toothed belts 30' are carried by the rollers 24, 28. A pair ofpressure rollers 104 are rotatably mounted above the belts 30' inrolling contact therewith. Thus, independently and at the proper time(as previously described), by stepping the motor 48, the control logic50 can independently turn the drive rollers 24, belts 30', idler rollers28, and pressure rollers 104 in combination to receive a sheet of paper12 from the sheet feeder (not shown) and push it horizontally toward thelower paper guide assembly 36. To assure proper paper movement on asheet-by-sheet basis since the paper is not continuous in thisapplication, it is anticipated that the lower paper guide assembly 36will need to include additional paper guiding provision such as theupper guide 106.

Another possible variation of the present invention which may bedesirable in some applications employing continuous form paper isdepicted in FIG. 9. In this variation, labelled as 32₁₁, the upperrollers 74, 76 are replaced by tractor feeds 10₁₁ operably connected tobe driven by the upper motor 80. Other types of drives as known in theart could also be substituted, if desired. Several other variationspossible within the general teaching of the present invention ashereinbefore described in its preferred embodiment are also worthy ofmention at this point. For one, since the amount of paper in the loop 98available for advancement by the upper drive is metered in advance, theoptical sensor 88 could be omitted if necessary. The reliability of theprinter with respect to accurate line feeding could, of course, bedegraded by such removal and, for that point, the inclusion of thesensor 88 is preferred.

Having thus described the present invention and its manner of operationin general, the specifics of a tested embodiment thereof as incorporatedinto a high speed shuttle printer by the inventor herein will now beprovided by way of example. The operation of the invention in the abovecontemplated mode is shown in FIG. 7. During the illustrated printingcycle, the lower stepping motor 48 advances the tractor feed mechanism10' approximately 5/72". Just prior to line feed time, the upper motor80 is accelerated to 50"/second equivalent paper speed. At impact, thefeed rollers 74 accelerate the paper 12 at 140 g's until the paper'speak speed is 50"/second, after which some slowing occurs and the paper12 moves at an average of 40"/second until the 5/72" feed loop isremoved. The upper motor 80 is then decelerated by paper friction.Without the backlash coupling mechanism 84, 25"/second equivalent paperspeed has been employed for a 5/72" line feed. Typical times involved in1/6" paper motion according to the above-described scenario with thebacklash coupling mechanism 84 are as follows:

Acceleration of paper to 50"/second=0.5 ms;

During this time, the paper moves 1/72"

Time to move the paper the remaining 11/72"=4.0 ms

Total Time=4.5 ms

Returning again to FIG. 3 with particularity, an alternative manner ofoperation of the above-described apparatus under the control of thecontrol logic 50 will now be described in detail. During or after theprinting of a line of text on the paper 12, the logic 50 steps the lowerstepping motor 48 to lift the paper 12 from the supply stack and advancea next line feed portion horizontally forward as indicated by the arrow52. At the commencement of this action, the paper is being grippedbetween the pressure edge 72 of the pressure plate 60 and the guide bar54 and, therefore cannot move forward along its path of travel beyondthat point. Accordingly, the paper between the tractor feed mechanism10' and the above-described point of gripping is forced into thejunction of the horizontal and vertical portions 62, 64 of the pressureplate 60 thus initiating the formation of a line feed loop of the paper12 as indicated by the dashed line 98. The paper 12 is only advanced onedot line feed distance by the motor 80 and control logic 50.

When it is time for a line feed of the paper 12 following the end ofprinting of a line of text, the control logic 50 steps the upperstepping motor 80 to lift the small mass of the paper 12 between thegripping point of contact between the rollers 74, 76 and the line feedloop 98 through the gripping force which exists between the pressureedge 72 of the pressure plate 60 and the guide bar 54. Because of thelow mass being moved, there can be no overshoot and the paper 12 comesquickly to its new position as depicted by the solid lines in FIG. 3with the paper 12 stretched across the platen.

In this alternative manner of operation the control logic 50 initiatesthe stepping of the upper stepping motor 80 after the lower steppingmotor 48 has commenced advancing paper 12 but before the lower steppingmotor 48 has completed the advancement of the next line feed portion tothe line feed loop. The control logic 50 times the operation of thelower stepping motor 48 so that advancement of the entire next line feedportion of the paper 12 to the line feed loop is completed no later thanthe completion of the lifting of the entire next line feed portion ofthe paper 12 from the line feed loop past the pressure edge 72 of thepressure plate 60 and the guide bar 54. By virtue of this, the line feedloop, in this alternative manner of operation, never, in normaloperation, contains an entire next line feed portion of paper 12 and themass of paper to be lifted by the upper stepping motor 80 is therebyreduced.

Thus it can be seen that the present invention has met its statedobjective by providing a paper advancing apparatus and associated methodof operation which will advance paper line-by-line quickly andaccurately in very high speed printing operations.

We claim:
 1. A paper advancing mechanism for a printer, for printinglines of text, including a horizontal platen and a printing mechanismdisposed adjacent the platen for printing lines of text during side toside motion thereof with an interval between the printing of successivelines during which the printing mechanism reverses direction, the paperadvancing mechanism being for intermittently moving paper on aline-by-line basis, an exact length from a supply input, between theplaten and printing mechanism comprising:a) powered first drive meansdisposed before the platen and printing mechanism for receiving andgripping the paper from the supply input and for advancing an exactlength of paper for exactly one or a multiple of one line feed distanceto a supply loop between said powered first drive means and the platenand printing mechanism subsequent to commencement of the printing of aline by said printing mechanism; b) powered second drive means disposedafter the platen and printing mechanism for gripping and rapidly movingthe paper only said exact length from said supply loop between saidpowered first drive means and the platen and printing mechanism during asingle said interval, said powered second drive means being inoperativeto move said paper, while said printing mechanism is printing a line;and, c) control logic means operably connected to said powered firstdrive means and said powered second drive means for activating saidsecond drive means to move all of paper from said supply loop thereby toadvance said exact length of the paper past the platen and printingmechanism when the printing mechanism has completed printing a line andrequires the paper to be so advanced in preparation for the printing ofthe next line and for activating said first drive means, to receive andadvance only said exact length of paper from the supply input to saidsupply loop subsequent to the printing mechanism commencing the printingof said line and in time to enable the moving of said exact length ofpaper from said supply loop by said second drive means, whereby saidsecond drive means has only to move the minimum mass of paper needed forthe advance of said exact length.
 2. A paper advancing mechanismaccording to claim 1 comprising:frictional gripping means disposedadjacent the platen between the platen and said first drive means forfrictionally gripping the paper under a gripping force sufficient toprevent advancement beyond said frictional gripping means by said firstdrive means while permitting the paper to be pulled through saidfrictional gripping means by said second drive means.
 3. The paperadvancing mechanism of claim 1 comprising frictional gripping meansdisposed adjacent the platen between the platen and said first drivemeans for frictionally gripping the paper under a gripping forcesufficient to prevent advancement beyond said frictional gripping meansby said first drive means while permitting the paper to be pulledthrough said frictional gripping means by said second drive meanswherein, said second drive means provides sufficient drive to overcomethe frictional grip of said frictional gripping means while providinginsufficient drive to overcome the combined resistance, to movement ofthe paper, provided by the said first drive means and said frictionalgripping means.
 4. A paper advancing mechanism according to claim 1wherein said second drive means includes frictional slip means providingsufficient frictional drive for said rapid moving of the paper from thesupply loop between the platen and printing mechanism while providinginsufficient frictional drive to overcome the resistance, to movement ofthe paper, provided by said first drive means.
 5. A paper advancingmechanism according to claim 1 wherein said control logic meansactivation of said second drive means occurs while said first drivemeans is advancing said exact length of paper to said supply loop andsaid activation of said first drive means is timed to ensure that saidadvancement of said exact length of paper to said supply loop iscompleted no later than the completion of the advancement of said exactlength of paper past the platen and printing mechanism.
 6. A paperadvancing mechanism according to claim 1 wherein said control logicmeans is operably connected to said powered first drive means and saidpowered second drive means for activating said first drive means, toreceive only exactly the paper from the supply input required to formsaid supply loop, during the time the printing mechanism is printing aline and for activating said second drive means to move the paper saidexact length of said supply loop when the printing mechanism hascompleted printing a line and requires the paper to be advanced saidexact length in preparation for the printing of the next line.
 7. Apaper advancing mechanism according to claim 1 wherein said poweredsecond drive means comprises:a) a pair of feed rollers mounted forrotation and disposed parallel to the platen adjacent respective sideedges of the paper; b) a pair of pressure idler rollers mounted forrotation and disposed parallel to respective ones of said feed rollersin contacting relationship thereto; c) means for urging said pressureidler rollers against said feed rollers to create a gripping forcetherebetween for frictionally gripping the paper under a gripping forcesufficient to pull the paper through said frictional gripping means; d)a drive motor operably connected to said feed roller and said controllogic means; wherein said feed roller is concentrically attached to theend of a drive shaft of said drive motor to rotate in combinationtherewith; and e) an electromagnetic brake disposed between said feedroller and said end of said drive shaft of said second drive motor.
 8. Apaper advancing mechanism according to claim 1 wherein said poweredsecond drive means comprises:a) a pair of feed rollers mounted forrotation and disposed parallel to the platen adjacent respective sideedges of the paper; b) a pair of pressure idler rollers mounted forrotation and disposed parallel to respective ones of said feed rollersin contacting relationship thereto; c) means for urging said pressureidler rollers against said feed rollers to create a gripping forcetherebetween for frictionally gripping the paper under a gripping forcesufficient to pull the paper through said frictional gripping means; d)a drive motor operably connected to said feed roller and said controllogic means; wherein said feed roller is concentrically attached to theend of a drive shaft of said second drive motor to rotate in combinationtherewith; and e) a backlash coupling mechanism having an arc offreeplay motion disposed between said feed roller and said end of saiddrive shaft of said second drive motor; and f) said control logic meansincludes means for backing up said drive motor to a point towards thebeginning of said freeplay during the time the printer is printing aline of text and for starting said second drive motor in a forwarddirection at a time prior to the time said upper drive means is to movethe paper from said supply loop said feed distance such that saidfreeplay is exhausted at the time said upper drive means is to move thepaper from said supply loop said feed distance.
 9. A paper advancingmechanism according to claim 1 wherein said powered second drive meanscomprises:a) a pair of feed rollers mounted for rotation and disposedparallel to the platen adjacent respective side edges of the paper; b) apair of pressure idler rollers mounted for rotation and disposedparallel to respective ones of said feed rollers in contactingrelationship thereto; c) means for urging said pressure idler rollersagainst said feed rollers to create a gripping force therebetween forfrictionally gripping the paper under a gripping force sufficient topull the paper through said frictional gripping means; d) a drive motoroperably connected to said feed roller and said control logic means; ande) said feed roller is resiliently slip mounted on a shaftconcentrically attached to the end of a drive shaft of said drive motorto rotate in combination therewith.